Pilot valves for sensing a fluid pressure and shifting a valve element in response thereto have been widely employed in the prior art. A common form of such a valve is the "block and bleed" type, which employs a pressure balanced spool valve which is shiftable in response to a change in a monitored fluid pressure to apply or exhaust a control pressure, so that an actuator is controllably responsive to the monitored fluid pressure. The spool valve is normally urged to one position by a spring whose force is opposed to the monitored fluid pressure, with the spring acting on a piston operatively connected to the spool valve. This type of pilot valve is shown, for example, 1 in U.S. Pat. 4,420,011 to Roger.
Manually manipulatable pneumatic or hydraulic threeway normally closed relay valves are well known in the art. Such a relay may typically be used to control the fluid pressure to an actuator of a flow control valve, and includes a control inlet port, an outlet port, and a vent port. Control pressure is either passed from the control inlet port to the outlet port and thus the actuator, or is blocked to the vented outlet port, depending on the axial position of a valve member. The valve member is movably responsive to pilot signal pressure to the relay valve generally input via a lower pilot port, while manual intervention of the relay is obtained from an upper control knob movable with the valve member.
Normally there is a sufficient pilot signal pressure to the relay valve to overcome a spring and keep the relay open, thus transmitting control pressure from the inlet port to the outlet port. If the pilot signal pressure to the relay is between the inlet port and the outlet port. The relay thereafter stays closed, regardless of a change in the pilot signal pressure, until the operator manually reopens the relay, at which time the presence of a sufficient pilot signal pressure keeps the relay open. This type of relay is disclosed, for example, in U.S. Pat. No. 3,877,484, which is hereby incorporated by reference.
Such prior art relay valves typically require a multiple diameter bore within the relay body to control sealing engagement between a seal on the piston and the valve body. To keep the relay closed, the pilot signal port is blocked, typically by a metal-to-metal seal, so that this pressure is not acting upon the piston. Should this metal-to-metal seal leak, a gap around the seal is intended to permit the leaked fluid to bypass to the vent port. One disadvantage of this technique concerns relay reliability when the valve stem is intermediate between the relay open and relay closed positions. In this intermediate position, fluid communication is intended between the pilot signal port and the vent port, so that pilot signal pressure is bled to the atmosphere. When more than one relay is utilized with the same pilot pressure signal, this escape of fluid when one relay is in the intermediate position could result in a sufficient decrease in pilot signal pressure to inadvertently close other relays intended to be held open by the pilot signal pressure.
Another disadvantage of the above relay is that the pilot signal pressure still acts on a relay which is closed, and may be sufficient to inadvertently open the relay (thereby creating significant safety concerns) should other components of the relay fail. For example, if the relay is mounted "upside down" and the piston return spring were to fail, the return of a high pilot signal pressure acting upon the piston projection blocking at the pilot signal port could inadvertently reopen the relay. Even though this pilot signal pressure subsequently bypasses the piston and is vented, a pressure differential across the piston could still open the relay.
A desirable feature of a three-way relay as described above is that, once the relay closes due to a lack of pilot pressure, the relay cannot be reopened by the mere presence of pilot pressure. Typically, the pneumatic or hydraulic logic system utilized intends that operator intervention, normally applied by pulling up on the valve member, be required to reopen the relay, and further requires that the pilot fluid pressure be maintained at a sufficient level to keep the relay open. Under certain circumstances, however, it is not desirable to allow the relay to be opened only by manual intervention. In the case of an offshore oilwell, for instance, a relay of the type described above is normally desired, but after an emergency storm situation, it may be desirable to open the relay without requiring the time and expense normally associated with operator involvement to manually open a remotely positioned relay, which may be in an offshore or other remote environment.
The disadvantages of the prior art are overcome by the present invention, and an improved manual relay valve is hereinafter described.